KILNS (Pottery, for artists)
CHAPTER XV KILNS "By many long, laborious, and chargeable experiments he hath found out." Extract from an old potter's patent. WHAT must be the first representation of a kill, or kiln, is found at Beni Hassan. It appears to be square in form, and the potter is shown feeding the fire at the base. In the same illustration he is depicted unpacking or drawing the ware from the top. The cut from the Greek Hydra gives a very 151 152 similar kiln, but a vessel in the museum of Berlin shows one with a beehive shape. The kilns left scattered about Europe by the Romans were usually of this domed kind, circular in plan, with one fire hole. The floor of the kiln was of pierced slabs, and the flames issuing thence enveloped the ware piled within and escaped through a vent in the top. The packing and firing is de- scribed in Ceramic Art in Great Britain, by L. Jewitt, F.S.A. It fully explains the trepidation of the old potters, who, before each firing, were wont to consult the moon and stars and evoke the aid of the gods. This is happily set forth in Cowper's translation of one of Homer's epigrams, wherein he expresses the pious hope that if the false potter "stoops to peep into his furnace, may the fire flash in his face and scorch it"; a risk often faced by potters, false or true! The smothering, or reducing, as then practised, was similar to the lustring methods used in Italy in the sixteenth century, or in the manufacture of the blue bricks to-day. The Japanese and Chinese built small kilns in tiers on the side of a hill. Start- ing with the lowest, the waste heat was utilized to warm up the kiln above, thus saving time and fuel. The Chinese used heavy saggars, and specimens of these with portions of melted pots still adhering to them attest the enormous heats to which they frequently attained. 153 Modern kilns subdivide roughly into biscuit, glost, and enamel. The first is used for firing the green or clay shapes, the second for the hard fire of the glazed ware, and the last-named for fixing on the added decoration. Sometimes a kiln is used for the double function of biscuiting and glazing. Of modern kilns the one still most widely used approximates to the bottle-shaped, simple, up-draught kiln. It contains one or two chambers with hatch for entry, flue or chimney, and anything from three to nine fire holes. The section of such a kiln is here shown and represents a fair average up-draught kiln (Fig. 66), variants of which type are working in most pottery districts to-day. In these kilns the flames rush in at the fire holes, play on the built-up bungs of saggars, and escape through the top vent. In a two chamber kiln, as sometimes used for porcelain, the glaze is put in the lower chamber to receive the hottest fire, the biscuit in the upper getting a gentle fire. Where the fire enters directly into the kiln in any large volume, bags or small chimneys are built up inside the mouths to save the saggars from the worst of the fire. Of late the single-chamber, down- draught kiln has come into favour, as it is easily packed and economical of heat. Bags of firebrick protect the saggars from the roughest fire and direct the flames to the crown of the chamber, from which point they descend to pass out through flues in the 155 POWNDRAIJGHT KILN 55? FIG. 67 floor of the kiln. (Fig. 67.) Biscuit ovens are often of this type, either domed or flat arched. The most recent innovations are gas and oil kilns. They require comparatively little manual labour in the firing, are easily regulated, and the waste heat is frequently used for secondary purposes. Their freedom from smoke and their economy of labour and money must eventually render the other types obsolete. These large kilns are strongly banded with iron supports designed to prevent too much loosening of the walls when expanded by the great heat. A very important point is the draining of their site, 156 FIG. 68 as large kilns tend to attract moisture, and the presence of steam in a kiln is productive of much damage. Enamel kilns are used for the final processes where the glazed pot is painted with over-glaze or enamel colours. They are of no great size and are made of fire-clay slabs or even iron plates when the enamel is soft. The flames play all round the muffle or fire- clay box during the firing, but no flame or fume is allowed access to the inside. (Fig. 68.) 157 In packing a kiln with biscuit or glaze much care, thought, and expedition have to be exercised. An experienced setter is essential in a factory if the pots are to have every chance in the fire, for all kilns vary and have their hot and cool corners. First, the floor is well bedded with quartz sand or flint that will not vitrify but will provide a good setting for the bottom saggars. These saggars are fireclay boxes, round or oval, rarely square, and without lids, in which the pots are placed. Piled one above another they form the bungs, the bottom of one supplying the top of that beneath. These bungs are built at intervals that permit the flames to penetrate between them and give a good even fire all over the chamber. In actual practice some parts of a kiln are hotter than others, and it is here that a good setter shows his capabilities, setting the thin wares in the softest places and putting the heavier biscuit or hard glaze in the hottest corners. With biscuit the setting is not difficult except where delicate or friable ware may need very careful bedding and propping. With clean saggars the biscuit may touch the sides, and a competent man will pile an enormous amount of biscuit into a kiln without risk. In the glost kiln the ware requires gentle handling and must touch nothing but the spur or support. The saggars are usually given a saggar wash of lead and stone to prevent them absorbing glaze from the wares, for 158 a glazed pot placed too near a raw saggar is very likely to come out with a thin or dry patch. When the pot is firmly placed on its stilt, a roll of pugging is placed round the rim of the saggar ; this provides a firm bed for the next above it and also stops the entry of dust and flame. This pugging is made from clay or marl mixed with ground sherds, sieved gravel, or some non-vitreous dust to prevent it sticking to the saggars. The plugging is made malleable with a little water and rolled out by hand or pressed through a die. It will readily be seen that a carelessly built bung of any height may slip in the stress of firing, and its fall would most likely involve others, whilst any slight movement may be sufficient to cause a vase to topple off its stilt. In a down-draught kiln the bungs over any vent must be raised on fire tiles to permit the escape of the flames. When all the bungs are filled up and piled in position, the trials and cones placed, the hatch is bricked up. Spy holes are left where necessary, and the whole well clammed to prevent the loss of heat during firing. When fired, the hatch may be very gradually loosened to accelerate the cooling. The fireman's job is one of the most arduous and important of all the prolonged processes of potting. Coming as it does at the end, it may make or mar kilns of great value, and it requires more than ordi- nary judgement and nerve. The chief troubles arise 159 POWN-DRAQGHT KILN rOOTINCJ. 1- HEARTH SECTION ON J.RON BEAKED 4. 5TOPHK: J.HKEHOLE- 7. HATCH- ir o r r - ' j> HALf-PlAN FIQ. 69 160 from bad or irregular draught or impure fuel. They express themselves in the form of clinkered holes, excessive smoke, and irregularly fired or sulphured ware. The term sulphured is applied to pots that come out gloomy or dulled and is said to be equally due to a reducing fire or an oxidizing one. In the former an insufficiency of air causes excess of carbon in the kiln and the absorption of oxygen from the glaze. In the latter an excess of air (oxygen) allows sulphur vapour, if present, to attach itself to the glaze. The fire bars, fire holes, and flues must be kept clear and the fuel carefully selected if these things are to be avoided. The aim, as previously stated, is a gradually increasing fire, sharper towards the finish. For this purpose some potters finish off with wood which gives a long flame free from sulphur and clears the glaze. Even when sulphured, a clear flame at the finish will remove many ill effects. The baiting should be fairly heavy at the start, lighter and more frequent towards the finish, when smoke in any quantity should be avoided. For temperature gauges, the cones previously de- scribed are used and should be placed in an average place, screened as much as possible from draught or flame. Small sight pots and glaze rings placed near the spies can be hooked out and examined and are valuable aids, but their exposed position and the corrosive action of the flames must be taken into 161 account. Pyrometers are also used, but with all these aids the experienced fireman pays due regard to sight and colour. For a craftsman, the kilns that have been de- scribed are unsuitable on account of their size, if they are not altogether beyond his means. But a kiln of some sort is indispensable to a potter. The craftsman is nothing if not inventive. Small trials can be fired in crucibles in an open fire or on a gas ring. I have heard of some preliminary success with a gas cooker, discounted later by a frontal attack from the cook. Considerable knowledge can be gained from small, easily constructed, experimental kilns. The small trial kiln shown (Fig. 70) was constructed with a stout old crock, over which was built a core of bricks plastered with marl. The fire was started at each corner at the bottom, and when well alight, fed from the top with coke. A small spy at one corner closed with a piece of biscuit gave some idea of the progress of the firing. This, of course, had to be practically rebuilt at each firing, but as a makeshift was quite satisfactory. CKDCKSHTH JPV-HOU . CONL IN/IPE FIG. 70 162 CROCK M1CKFD CMfc VLA/TERLD WITH MAKL The gas and oil kilns described in Chapter IX are excellent as far as they go, and indeed the only practicable kind for indoor schools. But their restricted size soon be- comes irksome to a crafts- man, whilst the expense of firing makes only the finest work remunera- tive. For over-glaze work they are excellent, but for some reason glazes fired in them seem to lack some of the rich- ness and maturity the same glazes exhibit when fired in the slower and more soaking fire of a brick kiln. The dug-out kiln here depicted (Fig. 71) would be quite suitable for summer schools or for a crafts- man making soft peasant pottery. The section and sketch will indicate its construction. The materials are hard bricks and stout old boiler plates, or sheet iron. To pack or unpack, the middle section of the roof would have to be removed each time, and all glazed pots would need protection from scalings and gravel from above. The roof will sag at any big heat, and if of thin iron, will need propping. The firing would be done with soft coal or wood ; a very slow start, with a brisk draught and a long flame at the finish. FIG. 70s 163 IRON ROOF. INDUCL DRAUGHT 'DUGOVT* KILN- op COMMON REDBRICK. BRICK/ TO KKLABOS/t fUOOR,. FIG. 71 Of course the front will be hotter than the back, but if saggars can be obtained, the glaze may be placed in them to the front with the biscuit protected behind. Clay shapes fired up at the front may bend towards the fire, and any broken crocks should be used to screen them. The making of rough saggars is not difficult if a supply of fireclay is to hand, or clay and grog will serve at a pinch. The clay is rolled out and the saggar stuck up, much as described on page 32. 164 EAKTH WLLWMMU SECTION OF DUG-OUTKILN FIG. 71* Every joint must be carefully welded and the whole thoroughly dried. Then they are fired up in the kiln, very gently at first, and carried up to a temperature considerably above that which they will be subjected to when in use. For small trial crucibles ordinary clay mixed with pitchers and powdered coke proves satisfactory; the coke when fired out renders the body porous and the heat penetrates more swiftly. To construct the muffle kiln shown at Fig. 72 the aid of a competent bricklayer would probably be required. In this kiln glaze and biscuit would fire up without saggars, but will take rather longer. The plan and elevation of this particular kiln are given with all reserve. The design would probably 165 FIQ. 72 require considerable adjustment and modification before complete satisfaction was obtained. A down-draught kiln, although more difficult to construct than an up-draught, is more economical in the end and does its work more evenly. For the Benefit of those that may like to build a small and moderately cheap kiln drawings are here given. This Jdln was built by me when I had very little practical experience of kilns other than gas, but it served its purpose well. It was not banded, but this is essen- tial if the kiln is to stand hard and frequent fire. Firebricks were used for the fireholes, flues, floor, core, and dome; strong stock bricks for the rest. The glazed ware, which was fired harder than the biscuit, was saggared in bungs as usual up to about five feet; the biscuit piled on top and protected by old saggars and cracked pots from the roughest 166 /HLW1NC ARKANGE. -MENTOF Fio. 72a fire. There were no bags in this kiln, but the saggars used were very strong and had stood a much greater heat than that to which they were subjected in this kiln, so that they showed very little wear or tear after twenty firings. The stack of sixteen feet gave a good sharp draught, increased if necessary by the addition of an iron chimney and regulated by an iron damper. A wind screen or hotel is advisable for rough nights, and some sort of roof is necessary to protect the crown from the weather. It is impor- tant to have the arch of the hatch very strongly built, as it has to stand a lot of strain, and an iron support 167 MQFFLE KILN KJILT EIRE MICK/ AND CQ5M&ON RED/- HATCH HICKLD Up. TWO j? HOLL/. Fia. 72b too near the fire soon corrodes and needs replacing. Another essential is that the site be as dry as possi- ble and the foundation made solid with concrete; otherwise even a small kiln is liable to settle and crack. With a little extra expense a kiln of this kind could be banded round the impost and fire- holes, thus considerably prolonging its life. 168 POKER: ./LICE- FIG. 73 In firing this kiln about half a ton of coke and three quarters of a ton of good hard coal, giving a long flame, was used. Coke for the slow fire was first started on the bottom and maintained for sixteen or eighteen hours, lifted up on to the bars for another six or eight hours, when the saggars would begin to show signs of colour. The coal fire was then started about the 24th or 25th hour and con- tinued another 18 hours, more or less, according to the varying conditions, making in all some 40 or 45 hours. This gave a very evenly graduated heat from cone 1 at the base to cone .03 at the top. Bags were tried experimentally, but whilst giv- ing a more uniform heat, took much longer to fire up. At the finish of the firing the fireholed were bricked up, the damper closed when the fires died down. In about 24 hours the vent and the hatch were eased a little at 169 the top, and in 48 hours it was pulled down and the drawing commenced. Packed with the hard glazes SAGGAR POTINSIDL FIG. 74 at the bottom and the soft at the top this kiln an- swered excellently for the purposes for which it was required. Categoría:Pottery, for artists, craftsmen & teachers